Physics in blender 2.8 - can't interact with objects while simulation is running
I guess you looked deeper into this by now but as I'm also trying to achieve the same thing as Ben Clark might as well post my findings here. 

I've done some messing and discovered that providing the object is set to be a passive rigid body you can grab it, move it where you want it and then reset the sim (timeline) and it will react correctly.

https://gyazo.com/987177ca2f87fef851519d8e12be3511

To me this looks like something to do with the physics caching system, maybe someone knows how to turn that off? :)

For a quick and dirty workaround, fascinatingly real-time grab physics still works with rigid body constraints.
Wouldn't be helpful if you want to just wack a cube wall with another cube though...

https://gyazo.com/75cdc85da98a0e7a3f67175c4386c9ee

Cheers!